1. Field of the Invention
The present invention relates to an optical device configured so as to maintain the temperature of an optical element as constant as possible. More specifically, it relates to an optical device configured so as to maintain the temperature of a virtually-imaged phased array (VIPA) optical element as constant as possible in an optical device using a VIPA optical element for producing wavelength dispersion.
2. Description of the Related Art
In the conventional optical fiber communication system for optically transmitting information, a transmitter transmits optical pulses to a receiver through an optical fiber. However, the wavelength dispersion in an optical fiber, which is also called “chromatic dispersion”, degrades the quality of signals in the system.
More specifically, the result of wavelength dispersion shows that the transmission speed of signal light in an optical fiber depends on the wavelength of the signal light. For example, if an optical pulse with a long wavelength (for example, an optical pulse with a wavelength indicating red color) propagates faster than a short wavelength (for example, an optical pulse with a wavelength indicating blue color), such dispersion is called normal dispersion. Conversely, if an optical pulse with a short wavelength (for example, a blue color pulse) propagates faster than a long wavelength (for example, a red color pulse), such dispersion is called abnormal dispersion.
Therefore, if signal light that is transmitted from a transmitter consists of a red pulse and a blue pulse, the signal pulse is divided into the red pulse and blue pulses while it propagates through the optical fiber and they are received by the receiver at different times.
If as another example of optical pulse transmission, a signal light pulse with continuous wavelength components in which a red color component follows a blue color component is transmitted, the propagation time of the signal light pulse in the optical fiber prolongs and distortion occurs in the signal light pulse since each component propagates through the optical fiber at a different speed. Since each pulse consists of a limited number of wavelength components in a specific wavelength range, such wavelength dispersion is very common in an optical fiber communication system.
Therefore, particularly in a high-speed optical fiber communication system, it is necessary to compensate for wavelength dispersion in order to secure a high transmission capacity.
In order to compensate for such wavelength dispersion, an opposite dispersion component that applies wavelength dispersion which is the reverse of wavelength dispersion caused in an optical fiber, is needed in an optical fiber communication system.
As such an opposite dispersion component, an optical device, including an optical element called a “virtually imaged phased array (VIPA)”, is proposed in Japanese Patent Application Nos. 10-534450 and 11-513133.
FIGS. 1 through 3 show a VIPA and an opposite dispersion component using the VIPA.
A VIPA optical element makes a plurality of segments of input light interfere with each other and generates light to be transmitted from the VIPA optical element. A dispersion compensation device that acts as an opposite dispersion component using a VIPA optical element comprises a reflection device returning light to the VIPA optical element and causing multi-reflection in the VIPA optical element.
The optical device being a dispersion compensator receives input light with continuous wavelengths in the wavelength range and generates output lights with continuous wavelengths each corresponding to each component included in the input light. This output light can be spatially distinguished from another segment of output light with other continuous wavelengths in the wavelength range (for example, propagating in a different direction). If this output light can be distinguished from another segment of output light by a propagation angle, it can be said that this optical device has angular dispersion.
A VIPA optical element is composed of a transmission area and a transparent plate. Light can transmit into and out of the VIPA optical element through the transmission area. The transparent plate contains the first and second surfaces.
The first and second surfaces are reflectors. The reflector on the second surface is semi-transparent, and has both a reflective characteristic and a characteristic of transmitting part of input light. This reflector can be generally obtained by forming a transparent dielectric multi-layer film on the transparent plate. However, the first surface reflector is a fully reflective film that reflects the entire input light. Although the fully reflective film on the first surface is also a multi-layer film, the number of layers of this fully reflective multi-layer film is larger than that of the semi-transparent multi-layer film on the second surface. Input light is received by the VIPA optical element through the transmission area and is reflected many times on the first and second surfaces of the transparent plate. Therefore, a plurality of segments of light transmits through the second surface. The plurality of segments of transmission light interfere with itself and generates a plurality of segments of output light each of which propagates in a different direction depending on its wavelength.
Input light has continuous wavelengths in a specific wavelength range and output light can be spatially distinguished from another segment of light with other wavelengths in the wavelength range. The reflection device can return the output light to the second surface in the completely opposite direction. Then, this plurality of segments of returned light transmits through the second surface and is inputted into the VIPA optical element. Then, the plurality of segments of returned light is multiply reflected in the VIPA optical element and outputted to another input route from the transmission area of the VIPA optical element.
The reflection device of the optical device returns output light in one order of interference of a plurality of segments of light each in a plurality of orders of interference that is outputted from the VIPA optical element, and does not return the other segments of output light that are in other orders of interference to the VIPA optical element. In other words, the reflection device returns only light corresponding to a specific order of interference to the VIPA optical element.
In this case, the reflection device comprises a reflection mirror. The surface shape of the mirror is formed in such a way that the optical device produces specific wavelength dispersion.
As described above, the VIPA has an angular dispersion function like a diffraction grading and can compensate for wavelength dispersion. In particular, the VIPA is characterized by large angular dispersion and can easily make a practical opposite dispersion component.
As shown in FIG. 1, light inputted from an input fiber is forwarded to a collimation lens 11 by an optical circulator 10. The collimation lens 11 converts light that spreads and propagates from the output hole of the optical fiber, into parallel light. After transmitting through the transmission area of a VIPA optical element 13, the plurality of segments of light that is paralleled by the collimation lens 11 is focused in a line in the VIPA optical element.
The light focused in a line is reflected on the reflective films provided on the surface of the VIPA optical element 13 many times. Since one of the reflective films is semi-transparent, part of the light is outputted little by little to a focus lens 14 while the reflection is repeated many times. A plurality of segments of light that is outputted while the reflection is repeated interferes with itself and forms a plurality of segments of light flux with a different propagation direction each depending on its wavelength. The focus lens 14 focuses the plurality of segments of light on a specific position on the surface of the reflection mirror 15. The plurality of segments of light reflected by the reflection mirror 15 is inputted to the VIPA optical element 13 again through the focus lens 14. The plurality of segments of light inputted to the VIPA optical element 13 again in this way, is outputted from the transmission area of the VIPA optical element 13 after repeating multi-reflection. Then, the plurality of segments of light is inputted to the optical fiber through a line focus lens 12 and a collimation lens 11 and is combined there. The plurality of segments of light inputted to the optical fiber is outputted from an output fiber through the optical circulator 10.
FIG. 2 shows how the VIPA optical element generates output light.
A plurality of segments of light focused in a line is inputted to the VIPA optical element from a line focus lens through the transmission area provided with an anti-reflection film. The plurality of segments of input light is multiply reflected in the VIPA optical element. However, if this bent and folded reflection light route is expanded, it becomes a virtually imaged phased array. Therefore, the plurality of segments of light outputted from a virtual image interferes with itself and is reinforced by the interference. Then, a plurality of segments of light is formed on a semi-transparent multi-layer reflective film and is outputted. Although the plurality of segments of light formed by this interference propagates in a direction where the constructive interference conditions are met. Since the constructive interference conditions vary depending on wavelength, a plurality of segments of light flux is formed in different directions for each wavelength. Therefore, the VIPA optical element shown in FIG. 1 corresponds to a diffraction grating with a large diffraction order, and each segment of output light propagates in a direction where the constructive interference conditions are met.
FIG. 3 shows the principle of wavelength dispersion compensation using a VIPA optical element.
As shown in FIG. 3, each segment of light focused on a reflection mirror located after a focus lens is returned to an arbitrary position according to a reflection angle determined by the shape in a focus position of the reflection mirror and is inputted to the optical fiber again on a route the reverse of that taken when it is inputted to the optical fiber the first time and is combined there. If as shown in FIG. 3, the reflection mirror is convex, light with a short wavelength is returned to an upper beam image, and its optical path length becomes longer than that of light with a long wavelength, and its delay increases. Therefore, in this case, the dispersion compensator can generate negative dispersion. Conversely, if the reflection mirror is concave, the dispersion compensator can generate positive dispersion. Since a dispersion compensator using a VIPA is configured in such a way that when returning, light takes the same optical path as that taken when propagating, the dispersion compensator can be used in line using a circulator.
In an optical device using a VIPA optical element to compensate for wavelength dispersion, an operating wavelength can be accurately adjusted by heating the VIPA optical element and controlling its temperature.
However, if temperature inconsistencies are caused in the portion of the VIPA optical element through which light transmits, due to an inappropriate heating method of the VIPA optical element made of a transparent plate, and varying temperature distribution is obtained, inconsistencies in thickness and refractive index of the VIPA optical element occur and the periodicity of a virtually imaged phased array is destroyed. Accordingly, the degradation of the optical characteristic, such as the increase of insertion loss, decrease of transmission band and the like, of the device is caused.
In order to avoid temperature inconsistencies in the light transmitting portion of the VIPA optical element, to maintain uniform temperature distribution and to efficiently control such temperatures, a means for maintaining the optical characteristic desirable is needed.